1. Field of the Invention
The present invention relates to a processing machine, such as grinding machine, lapping machine or polishing machine, useful for the processing of a surface of a plate type work-piece, e.g. silicon wafer, to which very precise flatness; parallelism and surface roughness are required. In move in detail, the present invention relates to the driving mechanism which drives the platen of the processing machine.
2. Description of the Prior Art
Lapping or polishing to obtain excellent surface precision and surface roughness on the plate type of work piece to which precise flatness and parallelism are required, has been accomplished using the following method. That is, a work piece is pressed to a rotatable platen of lapping or polishing machine, then lapped or polished by a rotating action of the work piece and platen under the constant supply of compound for lapping or polishing containing abrasives, and the surface roughness of the work piece is improved.
Recently, the requirements to improve the machining accuracy and the dimensional stability to a silicon wafer and a compound semiconductor wafer, which are the starting materials of integrated circuit (IC) or large-scale integration (LSI), and a magnetic disk substrate made of aluminium alloy or glass, are becoming more important. Accordingly, the requirements for the processing machine to improve the thermal stability against temperature changes, and to reduce the vibration of machine, which greatly affect the processing results, are also becoming more important, further, regarding other incidental conditions, more severe control is becoming necessary.
In general, these machines such as lapping machine, or polishing machine, have a platen of considerable thickness made of metal e.g., cast iron, copper, tin or low steel, or non metallic materials, e.g., ceramics or glass, and the platen and a work piece are rotated severally. The processing is carried out on the surface of the platen. The mechanism of these type machines is to process the surface of work piece by friction caused by rotating action of fine particles of abrasive which exist between the platen and work piece. Therefore, the rotating speed must not be effected by the change of friction force and must not cause vibration. In the case of these machines the platen is rotated by the driving motor arranged at the lower position of the housing, while, the work piece is rotated by the driving motor arranged in one or more carrier heads which are placed at the upper position of the machine. The rotating speed of each driving motor is reduced to the necessary rotating ratio by a decelerator, and conveyed to the platen by a belt or a chain.
The above mentioned method converts high speed rotation of an electric motor to low speed high torque rotation by a reduction gear, and, since the size of the decelerator is relatively big, the total size of driving mechanism containing conveying parts becomes big. Therefore, the size of machine becomes bigger and also the weight of machine becomes heavier. The place where the machine can be set up is limited, and the machine can not be considered as a handy type machine.
The above mentioned conventional machine has following problems. That is, the reduction gear will generate heat which is unique to the reduction gear, and especially in a case of high precision processing, it is necessary to prepare a shelter board made of adiabator to prevent the transmittance of generated heat to the platen, or to prepare a quenching ventilator, and these additional apparatuses encourage the tendency of large size and heavy weight of machine. Further, the effective diameter of platen is becoming bigger to meet the bigger size work piece and to satisfy the requirement to improve the productivity, and this tendency is becoming more remarkable. Furthermore, since the supplying line of lapping or polishing compound is placed on the upper position of the machine, the size of machine becomes bigger and more complicated.
Further, reduction gear type decelerator has a tendency to cause the vibration accompanied with the rotation, and by the influence of the vibration, not only surface roughness or TTV (total thickness variation) of processed surface of work piece is deteriorated, but also the problem that the accuracy of an end point detection, which detects the stock removal of work piece, is put out of order further, the trouble of causing noise is also mentioned. And, in a case of the decelerator based on mechanical mechanism, the periodical supply of lubricating oil by periodical maintenance is necessary, further the stains of machine and environment by leaked oil is mentioned as another big problem.
In a driving system which is furnished with a conventional motor, a reduction gear type decelerator and a power transmitting mean, since electric current intensity and putout, that is, load and power, are not in proportion to, each other, the positioning accuracy of a work piece on a platen before and after processing is not good, and it is pointed out as one of the big obstacles for the development of an automatic and continuous type machine. Therefore, in a conventional type machine, it is necessary to equip an additional position sensor and position corrector, which are one of the factors which make the machine complicated.